100th Anniversary of Macromolecular Science Viewpoint: The Past, Present, and Future of Stereocontrolled Vinyl Polymerization

The thermomechanical properties exhibited by synthetic macromolecules can be directly linked to their tacticity, or the relative stereochemistry of repeat units. The development of stereoselective coordination-insertion polymerization, for example, led to the discovery of isotactic polypropylene, now one of the most widely produced commodity plastics in the world. Widespread interest in controlling polymer tacticity has led to a variety of stereoselective polymerization methodologies; however, this area of polymer science has lagged behind when compared to the ability to control molecular weight, dispersity, and composition. Despite decades of advancements, many stereoregular vinyl polymers remain unknown, particularly those comprised of polar functionality or derived from renewable resources. This Viewpoint provides an overview of recent developments in stereocontrolled polymerization, with an emphasis on propagation mechanism, and highlights successes, limitations, and future challenges for continued innovation.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

Organocatalyzed Group Transfer Polymerization

In contrast to the conventional group transfer polymerization (GTP) using a catalyst of either an anionic nucleophile or a transition-metal compound, the organocatalyzed GTP has to a great extent improved the living characteristics of the polymerization from the viewpoints of synthesizing structurally well-defined acrylic polymers and constructing defect-free polymer architectures. In this article, we describe the organocatalyzed GTP from a relatively personal perspective to provide our colleagues with a perspicuous and systematic overview on its recent progress as well as a reply to the curiosity of how excellently the organocatalysts have performed in this field. The stated perspectives of this review mainly cover five aspects, in terms of the assessment of the livingness of the polymerization, limit and scope of applicable monomers, mechanistic studies, control of the polymer structure, and a new GTP methodology involving the use of tris(pentafluorophenyl)borane and hydrosilane.

Dinuclear silylium-enolate bifunctional active species: remarkable activity and stereoselectivity toward polymerization of methacrylate and renewable methylene butyrolactone monomers

Novel dinuclear silylium-enolate active species, consisting of an electrophilic silylium catalyst site and a nucleophilic silicon enolate initiating site that are covalently linked as single molecules, and their unique polymerization characteristics and kinetics are reported. Such unimolecular, bifunctional propagating species are conveniently generated from activation of ethyl- and oxo-bridged disilicon enolate (i.e., disilyl ketene acetal, di-SKA) compounds with [Ph(3)C][B(C(6)F(5))(4)]. Both the ethyl- and oxo-bridged dinuclear species are much more active for the polymerization of methyl methacrylate (MMA) than the mononuclear SKA-based active species, exhibiting an approximate rate enhancement by a factor of 12 and 44, respectively. The oxo-bridged silylium-enolate species is considerably more active and controlled than the ethyl-bridged one, with their differences being even more pronounced in polymerizing a renewable monomer, γ-methyl-α-methylene-γ-butyrolactone. The polymerization by the oxo-bridged silylium-enolate active species follows first-order kinetics in both monomer and silylium catalyst concentrations, indicating a unimolecular propagation mechanism which involves an intramolecular delivery of the polymeric enolate nucleophile to the monomer activated by the silylium ion electrophile being placed in proximity in the same catalyst molecule. Highly stereoregular poly(methyl methacrylate) (PMMA), with a syndiotacticity up to 92% rr, can be produced in quantitative yield using the oxo-bridged propagator at low temperature.